A semiconductor laser device is widely applied to a light source in an optical communication system, an optical information processing apparatus etc. In the semiconductor laser device, a vertical radiation angle of an output light beam is desired to be small and close to a radiation angle parallel to a plane of an active layer in that the light beam is coupled to a light incident end of an optical fiber in the optical communication system, and that a focusing or collimating lens having a low numerical aperture (NA) is used to provide an optical head of a low cost in the optical information processing apparatus and so on.
In general, a semiconductor laser device comprises a double hetero-structure, wherein a radiation angle of an output light beam having a radiating direction vertical to a plane of an active layer is determined by a thickness of the active layer and a refractive index difference between the active layer and cladding layers provided on both sides of the active layer. The semiconductor laser device, typically, comprises a first cladding layer, an active layer, a second cladding layer, a current blocking layer having an aperture of a stripe shape and a cap layer successively grown on a semiconductor substrate, a first electrode provided on the cap layer, and a second electrode provided on the back surface of the semiconductor substrate, wherein a refractive index n.sub.1 of the active layer is more than a refractive index n.sub.2 of the first and second cladding layers (n.sub.1 &gt;n.sub.2).
In the semiconductor laser device, a vertical radiation angle of an output light beam becomes small as a thickness of the active layer is decreased, and as a refractive index difference between the active layer and the first and second cladding layers becomes small. However, a bandgap energy difference between the active layer and the first and second cladding layers can not be controlled independently of the refractive index difference, because a refractive index of a semiconductor becomes small as a bandgap energy thereof becomes large in accordance with the Kramers-Kronig relation. The bandgap energy difference has a deep connection with a threshold current for a laser oscillation and its temperature dependence, and the refractive index difference has a deep connection with a vertical radiation angle of an output light beam. Where the bandgap energy difference is increased to improve the threshold current and its temperature dependence, the refractive index difference is inevitably increased to result in the increase of the vertical radiation angle.
As described before, the vertical radiation angle becomes small as the active layer thickness is decreased. However, where the active layer thickness is decreased, it is difficult to control the doping and diffusion of an impurity. As a result, a phenomenon called a remote-junction in which a p-n junction is formed inside the first cladding layer away from an interface between the active layer and the first cladding layer often occurs in accordance with an autodoping (diffusion of impurities) in the crystal growth. This phenomenon is remarkably observed in an InGaAsP/InP semiconductor laser device and an AlGaInP/GaAs visible light semiconductor laser device in which Zn is doped. In this point, the doping control becomes easy, where the active layer thickness is as thick as approximately 0.1 .mu.m. However, the active layer thickness of approximately 0.1 .mu.m is generally too thick to provide a low radiation angle of an output light beam.
Here, one type of a conventional semiconductor laser device which is described on pages 590 and 591 of "Appl. Phys. Lett., Vol. 22, No. 11, 1 June 1973" will be explained. The conventional semiconductor laser device comprises an active layer of a refractive index n.sub.1, first and second inner cladding layers of a refractive index n.sub.2 provided on both sides of the active layer, and first and second outer cladding layers of a refractive index n.sub.3 (n.sub.1 &gt;n.sub.2 &gt;n.sub.3).
In the semiconductor laser device, an optical waveguide mode is mainly determined by the two inner cladding layers and the two outer cladding layers when the active layer is thin, because a contribution of the active layer to the optical waveguide mode is low in a waveguide composed of the thin active layer and the respective two inner and outer layers. On the other hand, a confinement of carriers into the active layer is determined by a relation of the active layer and the two inner cladding layers. Consequently, an optical confinement is controlled separately from a carrier confinement.
The conventional semiconductor laser device, however, has a disadvantage that the separate confinement of light and carriers is not realized, where the active layer becomes thick. That is, the conventional semiconductor laser device is considered to be the same as the aforementioned typical semiconductor laser device in that a low radiation angle of an output light beam is not obtained in a state that a bandgap energy difference between the active layer and the two inner cladding layers is large, where the active layer is thick.